drivers/leds/leds-netxbig: make LEDS_NETXBIG depend on LEDS_CLASS

We call led_classdev_register/led_classdev_unregister in
create_netxbig_led/delete_netxbig_led, thus make LEDS_NETXBIG depend on
LEDS_CLASS.

This patch fixes below build error if LEDS_CLASS is not configured.

    LD      .tmp_vmlinux1
  drivers/built-in.o: In function `create_netxbig_led':
  drivers/leds/leds-netxbig.c:350: undefined reference to `led_classdev_register'
  drivers/leds/leds-netxbig.c:361: undefined reference to `led_classdev_unregister'
  drivers/built-in.o: In function `delete_netxbig_led':
  drivers/leds/leds-netxbig.c:313: undefined reference to `led_classdev_unregister'

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Acked-by: Simon Guinot <sguinot@lacie.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>